Semi-closed brayton cycle gas turbine power systems

ABSTRACT

A semi-closed combined cycle power system  100  is provided which can also convert an open combined cycle gas turbine  10  into a non-polluting zero emissions power system. The prior art open combined cycle gas turbine  10  includes a compressor  20  which compresses air A′ and combusts the air A′ with a fuel, such as natural gas. The products of combustion and the remaining portions of the air form the exhaust E′ which is expanded through the turbine  40 . The turbine  40  drives the compressor  20  and outputs power. The exhaust E′ exits the turbine  40  and then can optionally be routed through a heat recovery steam generator  50  to function as a combined cycle. According to this invention, the exhaust E′ is not emitted into the atmosphere, but rather is routed to a divider  110 . The divider  110  includes two outlets for the exhaust E′ including a return duct  120  and a separation duct  130  which both receive a portion of the exhaust E′. The return duct  120  routes a portion of the exhaust E′ back to the compressor  20 . Before reaching the compressor  20 , an oxygen duct  150  adds additional oxygen to the exhaust E′ to form a gas mixture C which includes CO 2  and steam from the exhaust E′ and oxygen from the oxygen duct  150 . This gas mixture C has characteristics which mimic those of air, so that the compressor  20  need not be modified to effectively compress the gas mixture C. The gas mixture C is compressed within the compressor  20  and routed to the combustor  30  where the fuel combusts with the oxygen of the gas mixture C′ and produces exhaust E′ which is substantially entirely CO 2  and steam. This exhaust E′ is routed through the turbine  40  and expanded to drive the compressor  20  and output power. The exhaust E′ exits the turbine  40  and is routed back to the divider  110 , preferably by way of a heat recovery steam generator  50  or other heat removal device, so that the semi-closed cycle operates as a combined cycle power system  100 . The divider  110  directs a portion of the exhaust E′ to a separation duct  130  which leads to a condenser  140 . In the condenser  140  the exhaust E′ is separated by condensation of the steam/water portion of the exhaust and removal of the remaining CO 2  as gas from the condenser  140 . The only exhaust from the semi-closed power system  100  is water and CO 2  from the condenser. The CO 2  exhaust is substantially pure and ready for appropriate further handling and disposal. Hence, no pollutants are emitted from the semi-closed power system  100 . The return duct  120  can include a partial condenser  210  to condense a portion of the steam within the exhaust E′. This condensed steam is then routed back through the heat recovery steam generator  50 , where it is converted to steam. This steam can be injected through a steam injection port  233  directly into the combustor  30  to enhance the power output and efficiency of a steam injection power system  200  variation of this invention.

FIELD OF THE INVENTION

[0001] The following invention relates to Brayton cycle power generationsystems which include a gaseous working fluid which is compressed,heated and expanded to generate power. More particularly, this inventionrelates to a semi-closed Brayton cycle power system which includes a gasturbine operating on a working fluid which is partially recirculated,the system eliminating emission of pollutants and emitting carbondioxide in an easily separated and recoverable form.

BACKGROUND OF THE INVENTION

[0002] Gas turbine power systems have become popular systems for meetingmodern society's power needs. Not only do gas turbines provide thrustfor most large aircraft, but they also have been adapted for use ingeneration of electricity in stationary power plants.

[0003] Gas turbines operate on the Brayton cycle and have a workingfluid, typically air, which remains gaseous throughout the cycle. Whilethe Brayton cycle can theoretically be closed so that the working fluidrecirculates, the vast majority of operational gas turbine power plantsoperate as an open Brayton cycle. In the open Brayton cycle commonlyfound in commercial stationary power plants, air is drawn into acompressor where its pressure and temperature increase. The temperatureof the air is then further increased by combusting a fuel, most oftennatural gas, in the air to produce a working fluid including air (minusthe oxygen which reacts with the fuel) and the products of combustion ofthe oxygen and the fuel (typically carbon dioxide and steam). This hightemperature high pressure working fluid is fed into a turbine where theworking fluid is expanded and its temperature and pressure decreased.The turbine drives the compressor and typically additionally drives agenerator for the generation of electric power. The working fluid isexhausted from the turbine in a simple open Brayton cycle.

[0004] Most operational stationary gas turbine power systems include asimple open Brayton cycle only as one part of a combined cycle.Specifically, because the working fluid still has a relatively hightemperature when exiting the turbine, this heat can be used, such as togenerate steam in a heat recovery steam generator before finally beingexhausted. The steam heated within the heat recovery steam generator canthen be utilized to drive a steam turbine, such as that found in anytypical closed Rankine cycle steam power plant. When operated as acombined cycle, the open Brayton cycle gas turbine and closed Rankinesteam turbine combine to most efficiently extract power from the fuelcombusted within the gas turbine (in some systems over 50% thermalefficiency).

[0005] While significant advances in compressor and turbine designs havegreatly increased the efficiency with which the gas turbine operates andhave increased the temperature at which the gas turbine can operate, thegas turbine has certain drawbacks. One drawback of the open Braytoncycle gas turbine is that the exhaust includes oxides of nitrogen(NO_(x)). NO_(x) is a pollutant which can only be emitted in compliancewith strict environmental regulations within the United States. Also,open Brayton cycle gas turbines emit carbon dioxide (CO₂) into theatmosphere. While emission of CO₂ is not currently regulated by theUnited States government, mounting scientific evidence has connected theemission of CO₂ with global warming and other negative atmosphericeffects. Numerous proposals are being evaluated for regulation of theemission of CO₂. Accordingly, a need exists for a way to eliminate theemission of NO_(x), CO₂ and other pollutants from gas turbines.

[0006] Techniques do exist for reduction of the emission of NO_(x) andthe elimination of CO₂ from open Brayton cycle gas turbine exhaust. Theexhaust can be scrubbed of a significant portion of the NO_(x) byvarious different processes applied to the exhaust to either convert(i.e. using ammonia) or separate the NO_(x) from the exhaust. Such“scrubbers” not only decrease the efficiency of the operation of the gasturbine, they are costly and also fail to remove all of the NO_(x) fromthe exhaust.

[0007] Large quantities of CO₂ are produced within the open Braytoncycle gas turbine as one of the major products of combustion of thenatural gas in air. This CO₂ is exhausted in gas form mixed with thelarge amount of nitrogen in the air which passes through the gasturbine. If removal of the CO₂ is desired, the CO₂ must first beseparated from the nitrogen and other gaseous constituents of theworking fluid (i.e. by chemical and/or physical absorption of the CO₂,or endothermic stripping processes for separating CO₂ from the exhaustgases). The CO₂ can then be used in industrial processes or can beeliminated, such as by pressurization and sequestration in undergroundor deep sea locations. While such CO₂ sequestration is a knowntechnique, significant energy is utilized in separating the CO₂ from thenitrogen, and hence the efficiency of the open Brayton cycle gas turbineis significantly decreased when CO₂ separation is required. Accordingly,a need exists for more efficient separation of the CO₂ from otherportions of the working fluid so that the efficiency of the gas turbineis not radically diminished.

[0008] Closed Brayton cycle gas turbines have been developed for certainspecific applications, such as for gas turbines which operate in nuclearpower plants. In the closed Brayton cycle a working fluid is provided(typically Helium in nuclear power plants) which remains separate fromthe heat source and which is recirculated from the turbine exhaust backto the compressor without ever being exhausted. The compressor typicallyneeds modification when the gas being compressed is changed. Because theworking fluid is not exhausted, it is not a source of atmosphericpollution. The heat source which heats the working fluid can be nuclear,solar, geothermal or some other form of renewable non-polluting heatsource so that atmospheric emissions are avoided.

[0009] However, if combustion of a hydrocarbon fuel with air is utilizedto heat the working fluid between the compressor and the turbine, theclosed Brayton cycle gas turbine will still have an exhaust whichincludes CO₂ and NO_(x). While renewable non-polluting heat sources suchas nuclear, solar and geothermal are effective, they suffer fromdrawbacks which have limited their ability to be fully competitive withhydrocarbon fuel combustion powered gas turbine systems. Other closedBrayton cycle or partially closed Brayton cycle gas turbine powersystems have been proposed which utilize a mixture of CO₂ and oxygen asthe combustion medium. For instance, see U.S. Pat. No. 5,724,805 toGolomb. While such CO₂ closed Brayton cycle gas turbine systems do keepnitrogen out of the combustor and so do not produce NO_(x), the highdensity of CO₂ makes it ill suited for use within a compressor which hasbeen designed for compression of air. Accordingly, a need exists for aBrayton cycle gas turbine which heats the working fluid by combustion ofa hydrocarbon fuel and which avoids emission of pollutants into theenvironment.

[0010] Another known technique for modifying prior art open Braytoncycle gas turbines is to inject steam into the combustor upstream of theturbine. When steam is injected into the combustor, the power output andthe efficiency of the open Brayton cycle can be enhanced. Variousdifferent prior art steam injection open Brayton cycles are disclosed byWilson and Korakianitis in The Design of High-Efficiency Turbo Machineryand Gas Turbines, Second Edition, 1998, Prentice-Hall, Inc. Forinstance, Wilson and Korakianitis cite one study by the General ElectricCorporation that their LM5000 gas turbine, when fitted with steaminjection and intercooling will experience a power increase from 34 MWto 110 MW and an efficiency improvement from 37% to 55%, compared to asimple Brayton cycle gas turbine power system with no associated Rankinecycle. Such steam injection open Brayton cycles typically do not operateas part of a combined cycle, but rather utilize the heat recovery steamgenerator to turn feed water into steam for injection upstream of theturbine. Hence, by steam injection, high efficiencies and high poweroutputs are provided without requiring a separate steam turbine andcondenser as required for a combined cycle.

[0011] Steam injection open Brayton cycles also suffer from numerousdrawbacks. Such cycles require feed water purification to keep themachinery in good working order. Water purification costs hence impedethe desirability of prior art steam injection open Brayton cycles. Also,prior art steam injection open Brayton cycles still produce NO_(x),carbon dioxide and other pollutants which are emitted into theatmosphere as with the non-steam injection open Brayton cycle gasturbine power systems described above.

[0012] The prevalence of open Brayton cycle gas turbines andparticularly combined cycle gas turbine power systems throughout theworld which are emitting large amounts of NO_(x) and CO₂ into theenvironment makes desirable the provision of a method and apparatus forretrofitting open Brayton cycle gas turbines in a manner which does notinterfere with the existing equipment but which eliminates emission ofnitrogen oxides, CO₂ and other pollutants into the atmosphere, so thatcapital costs associated with such retrofits can be minimized. Suchretrofits would additionally benefit from the use of a working fluidwhich matches the performance characteristics of working fluids in knownprior art open Brayton cycle systems so that optimum performance of thesystem components can be maintained.

SUMMARY OF THE INVENTION

[0013] The needs for pollutant emission elimination and gas turbineefficiency preservation identified above are met by the semi-closedBrayton cycle gas turbine power system of this invention and theassociated working fluids employed by this system. This power systemutilizes all of the major components of an open Brayton cycle gasturbine power system and optionally also the major components of acombined cycle power system. Additional equipment is added torecirculate at least a portion of the working fluid exiting the turbineand to cool the exhaust if necessary so that it passes back to thecompressor forming a semi-closed Brayton cycle.

[0014] Specifically, the semi-closed Brayton cycle power system of thisinvention includes a divider coupled to the turbine outlet of anotherwise known prior art open Brayton cycle gas turbine. The dividersplits the exhaust flow of the working fluid exiting the turbine output.A portion of the divider leads to a return duct which directs a majorportion (approximately 85%) of the turbine exhaust back to thecompressor inlet. The other portion of the divider leads to a separationduct which leads to a condenser having a condensate outlet and a gasoutlet.

[0015] An oxygen duct directs oxygen into the return duct so that oxygenis added to the portion of the turbine exhaust which passes from thedivider into the return duct. The oxygen entering the return duct ismixed with the exhaust therein so that the compressor inlet receives amixture of the turbine exhaust and the oxygen.

[0016] The compressor gas mixture typically includes three gases whichare mixed together. These gases include oxygen, steam (water vapor) andCO₂. The percentage of the gas mixture which each one of theseconstituents provides can vary. A preferred simple constitution of thegas mixture can be 13% wt oxygen, 39% wt water and 48% wt CO₂. Theseconstituent percentages can vary somewhat. Preferably, the constituentswhich form the compressor gas mixture are present at a ratio which isselected so that the gas mixture mimics the properties of air, which isitself a mixture of gases. At the preferred constituent percentagesidentified above, the various quantifiable physical properties of air(i.e. gas constant, specific heat, density, etc.) are closely matched.Hence, the compressor compresses the gas mixture in the same manner thatit compresses air without operating outside of its design limits.

[0017] The compressed gas mixture enters the combustor where natural gasor pure methane is combusted with the oxygen in the gas mixture. Themethane combusts with the oxygen in the gas mixture just as it wouldwith the oxygen in air. If sufficient methane is supplied to consume allof the oxygen in the gas mixture (a stoichiometric mixture ratio), theworking fluid exiting the outlet port of the combustor is entirely CO₂and steam. These gases have two sources, CO₂ and steam from the gasmixture entering the combustor and CO₂ and steam generated as productsof combustion of the oxygen and the methane.

[0018] This working fluid passes through the turbine and exits theturbine output as the exhaust. Because the exhaust is entirely CO₂ andsteam, no NO_(x) is present and no NO_(x) elimination equipment need beutilized. If the semi-closed cycle is optionally acting as part of acombined cycle, the exhaust passes through a heat recovery steamgenerator where it gives up heat to the steam in the “bottoming” Rankinecycle. The exhaust then exits the heat recovery steam generator andenters the divider. A portion of the exhaust is directed to the returnduct where it is directed back to the compressor. This exhaust, whenmixed with the oxygen from the oxygen duct returns to the appropriateproportions necessary to constitute the gas mixture described in detailabove. The gas mixture then again passes through the semi-closed Braytoncycle as described above.

[0019] A portion of the exhaust entering the divider is diverted intothe separation duct. This exhaust enters a condenser. Because theexhaust is entirely CO₂ and water, and because water condenses at a muchhigher temperature than CO₂, the condenser can very effectively andefficiently condense the water while the CO₂ remains gaseous. Acondensate outlet removes the water portion of the exhaust. The water ispure and can be utilized as desired without contamination of theenvironment.

[0020] The condenser gas outlet removes CO₂ from the condenser. This CO₂is essentially pure. Hence, the high energy process of removing CO₂ fromnitrogen which would be necessary to separate CO₂ from exhausts of priorart open Brayton cycle gas turbines is avoided. The CO₂ can be sold asan industrial gas, utilized beneficially or can be compressed andsequestered in an underground sequestration site, deep ocean location orany other suitable terrestrial formation.

[0021] Because the compressor gas mixture and other working fluids haveproperties which mimic those of air the major components of the openBrayton cycle gas turbine can be left unmodified and the remainingportions of the semi-closed Brayton cycle of this invention can be addedso that an open Brayton cycle gas turbine power system can beretrofitted and modified into a non-polluting power plant. Such aretrofit can occur both for a simple open Brayton cycle gas turbinepower system with addition of an appropriate turbine exhaust coolingheat exchanger and for a combined cycle power system.

[0022] The semi-closed Brayton cycle gas turbine power system of thisinvention can be adapted to utilize steam injection upstream of theturbine to provide the semi-closed Brayton cycle with the enhancedefficiency and power output benefits of steam injection detailed above.Because the semi-closed power system generates purified water, thisgenerated water source is used and a separate purified water source isnot required. Specifically, in the semi-closed cycle with steaminjection a partial condenser is located within the return duct whichcondenses some of the steam out of the exhaust. The water produced bythe condensation of some of the steam in the exhaust is routed throughthe heat recovery steam generator where it is converted back into steam.

[0023] This pure steam is then injected upstream of the turbine. Forinstance, the steam can be injected with the fuel, injected with theoxidizer from the compressor, or injected separately into the combustoror between the combustor and the turbine. While not preferred, excesswater exiting the condenser downstream from the separation duct couldsimilarly be utilized for steam injection.

[0024] When the steam is produced from water extracted from a partialcondenser in the return duct, the ratio of steam to carbon dioxidewithin the working fluid passing through the return duct is decreased.As a result, the compressor can compress more oxygen and less steam withthe same amount of work. With more oxygen passing through thecompressor, more fuel can be combusted in the combustor and the poweroutput of the semi-closed cycle is increased. Also, efficiency of thecycle is increased. While steam injection is typically utilized as areplacement for the “bottoming” Rankine cycle of the semi-closedcombined cycle, steam injection could be utilized within a semi-closedcombined cycle power system with the heat recovery steam generatorgenerating steam for injection into the combustor upstream of the gasturbine and also generating steam for use within the bottoming Rankinecycle.

OBJECTS OF THE INVENTION

[0025] Accordingly, a primary object of the present invention is toprovide a Brayton cycle gas turbine power system which does not emitNO_(x) or other pollutants, and which efficiently collects CO₂ forbeneficial use or elimination.

[0026] Another object of the present invention is to provide a Braytoncycle gas turbine power system which recirculates a portion of theturbine exhaust for input into the compressor of the power system.

[0027] Another object of the present invention is to provide a processfor modifying an open Brayton cycle gas turbine to function as asemi-closed Brayton cycle gas turbine power system which substantiallyeliminates emission of pollutants.

[0028] Another object of the present invention is to providesubstantially nitrogen free air substitute working fluids which canoperate within a Brayton cycle gas turbine without significantlyaltering the performance of the gas turbine and eliminate pollutantemissions from the gas turbine.

[0029] Another object of the present invention is to provide a powersystem which can efficiently generate power from the combustion ofhydrocarbon fuels without emission of pollutants.

[0030] Another object of the present invention is to provide asemi-closed gas turbine power system with steam injection to enhance thepower output and, efficiency of the power system.

[0031] In addition to the above objects, various other objects of thisinvention will be apparent from a careful reading of this specificationincluding the detailed description contained herein below.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0032]FIG. 1 is a schematic of a combined cycle gas turbine power systemsuch as that which is in common use for electric power generation.

[0033]FIG. 2 is a schematic of a semi-closed combined cycle gas turbinepower system according to this invention.

[0034]FIG. 3 is a schematic of a semi-closed gas turbine power systemwith steam injection according to this invention.

[0035]FIG. 4 is a schematic showing how the oxidizer gas mixture andexhaust gas mixture forms of the working fluid are created and routedthrough the semi-closed Brayton power system.

[0036]FIG. 5 is a schematic of a semi-closed Brayton cycle power systemfeaturing an air start-up mode.

[0037]FIG. 6 is a schematic of a semi-closed Brayton cycle power systemfeaturing integration with an ion transfer membrane (ITM) air separationunit (ASU).

[0038]FIG. 7 is a schematic of a combined cycle variation of that whichis shown in FIG. 6.

[0039]FIG. 8 is a schematic of an alternative combined cycle variationof that which is shown in FIG. 6.

[0040]FIG. 9 is a schematic of a semi-closed combined cycle featuring anion transfer membrane (ITM) air separation unit (ASU) and supplementaryheating in the bottoming cycle.

[0041]FIG. 10 is a schematic of a variation of that which is shown inFIG. 9.

[0042]FIG. 11 is a schematic of an air oxidizer gas turbine combinedcycle with an oxygen combustion supplementary heating system as thebottoming cycle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0043] Referring to the drawings, wherein like reference numeralsrepresent like parts throughout the various drawing figures, referencenumeral 10 is directed to a prior art gas turbine combined cycle powersystem which can be retrofitted according to this invention to become asemi-closed gas turbine combined cycle power system 100 (FIG. 2).

[0044] While many variations and enhancements to this invention arepossible, this invention is illustrated by the following detaileddisclosure of a simple conversion of a combined cycle gas turbine powersystem into the semi-closed combined cycle power system of thisinvention. This disclosure is believed to be the best mode for such asimple conversion. It is acknowledged that enhancements could be made toincrease the complexity and performance characteristics of theinvention. Such enhancements are not considered to be part of the bestmode for practicing the simple conversion of this invention.

[0045] This disclosure is also provided to enable one skilled in the artto perform the basic conversion of a combined cycle gas turbine powersystem 10 into the semi-closed combined cycle power system 100 of thisinvention or to construct the semi-closed combined cycle power system100 of this invention originally from the separate components making upthis system 100. Also, this disclosure is provided to disclose the bestmode for performance this simple conversion of the combined cycle gasturbine power system 10 into the semi-closed combined cycle power system100 of this invention in its simplest and most easily executable form.

[0046] In essence, and with particular reference to FIG. 2, thesemi-closed combined cycle power system 100 preferably includes all ofthe equipment contained within a prior art open Brayton cycle gasturbine, such as in the power system 10 shown in FIG. 1. The semi-closedpower system 100 adds a divider 110 to an exhaust of a turbine 40. Thedivider 110 diverts some of the exhaust to a return duct 120 which leadsto a compressor 20. Other portions of the exhaust are diverted to aseparation duct 130 which leads to a condenser 140. An oxygen duct 150introduces oxygen into the return duct 120 so that the compressorreceives a mixture of turbine exhaust and oxygen.

[0047] The compressor compresses this gas mixture and utilizes it withina combustor 30 to combust fuel, such as natural gas or pure methane. Thecombustor exhaust is then passed through the turbine where it isexpanded and does work, both driving the compressor and outputting powerfrom the power system 100. The exhaust is then directed back to thedivider 110 identified above.

[0048] While not necessary, maximum efficiency can be provided bypassing the exhaust through a heat recovery steam generator 50 betweenthe turbine 40 and the divider 110. The heat recovery steam generator 50transfers heat out of the exhaust E and into a steam heating circuitwhich leads to a separate steam turbine and generator 60 for additionalpower output from the power system 100.

[0049] This semi-closed power system 100 completely operates on mixturesof gases including CO₂, steam and oxygen. Portions of these gases whichare diverted by the divider 110 to the separation duct 130 can beexhausted into the atmosphere as none of these gases are currentlyregulated in the United States with restrictions on their emission aspollutants. However the exhaust, which is primarily CO₂ and water, caneasily be separated within a condenser so that pure streams of excesswater and excess CO₂ can be provided. These pure streams can then befurther handled in a manner other than exhaust into the environment ifdesired such as to avoid emission of CO₂, a greenhouse gas.

[0050] More particularly, and with specific reference to FIG. 1, detailsof the prior art combined cycle gas turbine power system 10 aredescribed which can be retrofitted to form the semi-closed power system100 of this invention. The combined cycle gas turbine power system 10 isa variation on the open Brayton cycle. Combined cycle gas turbines arein widespread use in the United States and are capable of particularlyhigh power output per unit size and particularly high efficiency. Asimplest open Brayton cycle gas turbine power system could also bemodified according to this invention by merely eliminating the heatrecovery steam generator 50 and steam turbine and generator assembly 60from the power systems 10, 100.

[0051] The combined cycle gas turbine power system 10 begins byinduction of air A into an inlet 22 of a compressor 20. The compressor20 increases a pressure of the air A, typically approximately tenfoldbefore the air A leaves the compressor 20 through the outlet 24. The airA is then directed to an oxidizer port 34 of a combustor 30. Thecombustor 30 additionally includes a fuel port 32. The fuel port 32delivers fuel, typically natural gas or pure methane, into theappropriate combustor 30. With modifications, other hydrocarbon fuelscan be used.

[0052] The oxidizer port 34 has air A passing therethrough and into thecombustor 30 when the combined cycle gas turbine power system 10 isoperating according to the prior art. While air can be considered to bean oxidizer, it is in fact the oxygen within the air which acts as theoxidizer. Other constituents of the air, primarily nitrogen, largely donot react within the combustor 30 and merely pass through the turbine 40as part of the exhaust E. However, the combustor 30 typically achievessufficiently high temperatures during combustion of the fuel that someof the nitrogen does react with the oxygen in the air to form nitrogenoxides, referred to as NO_(x). Hence, the exhaust E leaving thecombustor 30 through the outlet port 36 not only includes products ofcombustion of the fuel with the oxygen, namely CO₂ and steam, butadditionally includes the nitrogen in the air as well as NO_(x) andtypically some quantity of volatile organic compounds, referred to asVOCs, and particulates.

[0053] These various constituents of the gas mixture exiting thecombustor 30 through the outlet port 36 are together referred to as theexhaust E. The exhaust E is then directed to the turbine 40 through theinput 42. The exhaust E passes through the turbine 40 and exits theturbine 40 through the output 44. The turbine 40 includes a compressordrive shaft 46 or other mechanical, electrical or other power couplingbetween the turbine and the compressor 30 which allows a portion of thepower generated by the turbine 40, as the exhaust E expands through theturbine 40, to drive the compressor 20. Additional power 40 generated byexpansion of the exhaust E through the turbine 40 is outputted from theturbine 40, typically to an electricity generator along arrow 48.However, it is known in the prior art for the turbine 40 to be directlycoupled to a prime mover or transmission, such as a prime mover on aship or to an output nozzle on a jet engine, or to otherwise bemechanically or otherwise coupled to other devices for outputting ofpower other than to an electricity generator.

[0054] The exhaust E can be released into the surrounding atmosphereupon exiting the output 44 of the turbine 40. In such an arrangement,the gas turbine is functioning as a simple open Brayton cycle gasturbine power system, rather than as a combined cycle gas turbine powersystem 10. Preferably, however, in situations where efficiency is to bemaximized or where steam production is beneficial for heating or use inprocessing equipment which requires steam, the exhaust E is directed toa heat recovery steam generator 50.

[0055] The heat recovery steam generator 50 includes an entrance 52which receives the exhaust E and an exit 54 where the exhaust E isreleased into the surrounding environment. The heat recovery steamgenerator includes a steam heating circuit 56 therein which receivesheat which is transferred out of the exhaust E into the steam heatingcircuit 56. The steam heating circuit 56 is part of a steam turbine andgenerator assembly 60 which has a working fluid pathway 62 which passesinto the steam heating circuit 56 of the heat recovery steam generator50 before returning the steam working fluid back to the steam turbineand generator assembly 60. This assembly 60 would typically include acondenser and pump, as is known in the art to provide a closed Rankinecycle. An alternative to the assembly 60 is to merely provide the steamin the pathway 62 for other heating requirements (i.e. food processing,chemical processing, heating buildings, etc.).

[0056] The steam turbine and generator assembly 60 function as a typicalclosed Rankine cycle steam power plant except that rather than having afired boiler for boiling the water to make steam working fluid, thewater is passed through the steam heating circuit 56 of the heatrecovery steam generator 50 within the combined cycle gas turbine powersystem 10 to boil the water into steam working fluid before it isdirected into the steam turbine of the steam turbine and generatorassembly 60 or used for other heating purposes. In essence, the heatrecovery steam generator 50 and the steam turbine and generator assembly60 put to use the excess heat remaining within the exhaust E so that anoverall efficiency of the combined cycle gas turbine power system 100 isincreased.

[0057] The exhaust E which is directed into the atmosphere has the sameconstituents that it has upon exiting the combustor 30 through theoutlet port 36. Namely, the exhaust E is a majority nitrogen from theair A which was introduced through the compressor 20 and into theoxidizer port 34 of the combustor 30. The exhaust E also includes largeamounts of CO₂ and steam which were generated within the combustor 30 bythe combustion of the methane, natural gas or other hydrocarbon fuelwith the oxygen in the air A. The exhaust E would typically additionallyinclude oxygen which was in excess of the amount necessary to combustall of the fuel.

[0058] The nitrogen, any excess oxygen and the steam are exhaust Econstituents which are already prevalent in Earth's atmosphere and donot impact the quality of the environment. The CO₂ within the exhaust Eis also a constituent of air in Earth's atmosphere, but is present asless than 1% of air. While CO₂ is a constituent of air, within Earth'satmosphere, evidence is mounting that detrimental environmental impactsare associated with increases in the CO₂ present within Earth'satmosphere. For instance, CO₂ is often referred to as a “greenhouse gas”which is believed to contribute to global warming.

[0059] CO₂ is not currently regulated in the United States as apollutant, but elimination of CO₂ from the exhaust E which is directedback into earth's environment from combined cycle gas turbine powersystems 10 is widely considered to be a desirable objective. Theremaining constituents of the exhaust are pollutants which in many caseshave emission thereof strictly regulated and which detrimentally impactair quality if released or which severely diminish efficiency ifcollected.

[0060] With particular reference to FIG. 2, the semi-closed combinedcycle power system 100 of this invention is described in detail. Thesemi-closed power system 100 begins with each of the major components ofthe combined cycle gas turbine power system 10 (FIG. 1) described indetail above. However, the exit 54 of the heat recovery steam generator50 does not direct the exhaust E′ directly into the surroundingatmosphere. Rather, a divider 110 is provided which receives the exhaustE′ exiting from the exit 54 of the heat recovery steam generator 50. Thedivider 110 has two outlets including a return duct 120 and a separationduct 130. The return duct 120 directs a major portion (approximately85%) of the exhaust E′ back to the inlet 22 of the compressor 20. Theseparation duct 130 directs a remaining portion of the exhaust E passingthrough the divider 110 to a condenser 140.

[0061] The divider 110 can have any of a variety of configurations. Theprimary feature of the divider 110 is that it is capable of splittingthe exhaust E′ flow into the two separate pathways including the returnduct 120 and the separation duct 130. In a simplest form of theinvention, the divider could be unadjustable and merely utilize pressuredifferentials between the return duct 120 and the separation duct 130 todivide the exhaust E′ flow between the return duct 120 and theseparation duct 130.

[0062] Alternatively, a pressure sensitive valve 132 of some sort couldbe used, such as by placement in the divider separation duct 130. Forinstance, if it is desirable for the return duct 120 to have a pressureslightly higher than atmospheric, the valve would be slightly closed toincrease the resistance of separation duct 130 and reduce the flow rate.This temporary reduction in the flow rate of separation duct 130increases the flow rate of return duct 120 and thus increases therecirculating mass of the system and corresponding pressure level. Whenthe proper pressure is achieved, the valve in separation duct 130 isreturned to its original position. To reduce the system pressure thereverse of these steps is performed.

[0063] Because the combustor 30 increases an amount of gas in thesystem, more exhaust E′ flows out of the turbine 40 than the amount ofgas that flows into the compressor 20. To keep the pressure within thereturn duct 120 at the desired level, such as near or slightly aboveatmospheric pressure, the separation duct 130 would have the valve 132within the separation duct 130 open sufficiently so that excess exhaustE′ would be diverted through the separation duct 130 and the desiredpressure within the return duct 120 would be maintained. Such a valve132 in the separation duct 130 would control the pressure of the systemby increasing or decreasing the flow rate exiting duct 130 until thedesired pressure has been reached and steady state conditions areachieved.

[0064] The control valve 132 could also be configured with a vane/damperwhich might automatically move to divert more or less flow from thereturn duct 120 and into separation duct 130 merely by pressuremonitoring. Also, the vane/damper 130 could be coupled to a drive axleand pressure sensors or other sensors for controlling the semi-closedpower system 100 could be utilized to actively control flow rates andpressures within the return duct 120 and the separation duct 130 bycontrolling the position of the vane/damper 130 within the separationduct 130.

[0065] The return duct 120 typically is in the form of a substantiallyairtight conduit. Because the return duct 120 includes exhaust E′therein which includes steam, it would typically need to maintainsufficient temperatures, approximately 180° F. to 200° F. so that thesteam making up a portion of the exhaust E′ does not condense within thereturn duct 120. The return duct 120 can be fitted with insulation toprevent condensation along the walls of the return duct 120.Additionally, or alternatively, the return duct 120 can be manufacturedfrom materials which can accommodate the condensation of small amountsof water out of the exhaust 120 and merely provide water traps atlowermost portions of the return duct 120 to collect any watercondensing within the return duct 120 and direct the water out of thereturn duct 120, such as into the condenser 140.

[0066] The return duct 120 additionally includes an oxygen duct 150which introduces oxygen into the return duct 120. The oxygen duct 150 iscoupled to a source of gaseous oxygen. Such a gaseous oxygen sourcecould be of a variety of types including air separation units which arecapable of removing oxygen from the air, a gaseous oxygen pipeline, aliquid oxygen tank under pressure which bleeds off gaseous oxygen intothe oxygen duct 150 or any of a variety of other oxygen supply sources.

[0067] Preferably, the oxygen duct 150 receives oxygen from an airseparation unit which is powered by power outputted from the turbine 40,so that the entire semi-closed combine cycle power system 100 includesthe air separation unit therein. The air separation unit can be of aliquefaction type which separates the oxygen from the air by cooling theair to below the condensation point for the nitrogen in the air so thatthe nitrogen is removed from the oxygen in the air. Such a cryogenic airseparation system is described in detail in U.S. Pat. No. 5,611,219 toBonaquist, incorporated herein by reference. Alternatively, the airseparation unit can be configured utilizing ion transfer membranetechnology such as that described in detail in U.S. Pat. No. 5,447,555to Ye, incorporated herein by reference. Other techniques such as vacuumpressure swing adsorption could similarly be utilized, such as thatdescribed in detail in U.S. Pat. No. 5,194,890 to McCombs incorporatedherein by reference, to separate oxygen out of the air and deliver theoxygen from the oxygen duct 150 into the return duct 120.

[0068] The oxygen from the oxygen duct 150 is introduced into theexhaust E′ within the return duct 120 to form a gas mixture C, called“C-gas” which mimics the characteristics of air. Specifically, the gasmixture C includes the products of combustion of the methane or naturalgas with oxygen, namely CO₂ and steam along with the oxygen introducedfrom the oxygen duct 150.

[0069] The ratio of the CO₂ with the steam is similar to that whichnaturally occurs when the methane or natural gas fuel is combustedstoichiometrically with oxygen. Specifically, this ratio of CO₂ to steamwhen methane is combusted in oxygen stoichiometrically is 55% CO₂ byweight and 45% steam by weight. This ratio of CO₂ to steam is maintainedwithin the exhaust E′, through the divider 110, along either the returnduct 120 or the separation duct 130 and into the compressor 20, wherethe exhaust E′ is combined with the oxygen from the oxygen duct 150 toform the gas mixture C. However, when the oxygen is mixed with theexhaust E′ including the CO₂ and the steam, the gas mixture Cadditionally includes the oxygen. While the ratio of CO₂ to waterremains the same, the ratio of CO₂ to the overall gas mixture C and theratio of the steam to the overall mixture of the gas mixture C arereduced by an amount proportional to the amount of oxygen introducedinto the return duct 120 through the oxygen duct 150. An example of thisgas mixture C is provided as follows.

EXAMPLE 1

[0070] In this example a minimum amount of oxygen is supplied by theoxygen duct 150 so that the methane fuel burns at a stoichiometric ratiowith the oxygen within the gas mixture C and no excess oxygen remains inthe exhaust E′. In this hypothetical example, the gas mixture C would beprovided with sufficient oxygen so that the gas mixture C would be 13%oxygen by weight.

[0071] Note that air is approximately 23% oxygen by weight and typicallyreacts with methane with some excess air present which is not necessaryfor combustion of all of the methane. The gas mixture C has the CO₂constituents and steam constituents proportionally reduced to providefor the inclusion of the oxygen as 13% wt of the gas mixture C.Specifically, the CO₂ is reduced from 55% wt of the exhaust E′ to 48% wtof the gas mixture C. The steam is reduced from 45% wt of the exhaust E′to 39% wt of the gas mixture C.

[0072] Various different formulations for the gas mixture C can beprovided, varying from the specific gas mixture C of Example 1, whileremaining within the basic concept of the semi-closed combined cyclepower system 100 of this invention. Other examples could certainly beutilized.

[0073] A serendipitous and remarkable attribute of the semi-closedcombined cycle power system 100 of this invention is that when oxygen isadded to the natural products of combustion of methane and oxygen, in anamount sufficient to replace the combusted oxygen, the resulting gasmixture C (having 48% CO₂, 39% water and 13% oxygen by weight) hasattributes which very closely mimic the attributes of air in earth'satmosphere. Hence, not only is the gas mixture C the most simplyachievable constituent makeup for the gas mixture C, but this gasmixture C also has air mimicking characteristics which allow it to bedirected into the compressor 20 and operate within the design parametersof the compressor 20 that has been originally designed and constructedfor the compression of air A (FIG. 1) rather than the gas mixture C. Asa result, the compressor 20 does not require modification to handle thegas mixture C rather than air A (FIG. 1).

[0074] Table 1 provides twelve quantifiable gas parameters which areimportant to the design of compressors such as the compressor 20. Inthis table the values for the parameters of the gas mixture C, proposedin the above example, are shown alongside the values for these variousparameters for air. A review of this table and a comparison between thegas mixture C and air shows that for most parameters the values for airand the values for the various gas mixture C are very close. The inletpressure of the semi-closed cycle 100 can be adjusted, as describedabove, to better match gas mixture C properties with those for air.Also, small compressor speed changes may be desirable. Such speedchanges can be done when the compressor design characteristics areknown, to fully optimize the cycle 100. TABLE 1 Parameter Gas Mixture¹Air Gas Constant - R, ft/° F. 56.6 53.3 Specific Heat - Btu/lb° F. 0.3360.250 Specific Heat Ratio (γ) 1.28 1.38 Typical Pressure Ratio 30 30Inlet Temperature (T₁), ° F. 200 80 Outlet Temperature (T₂), ° F.⁽²⁾1058 1065 Inlet Pressure (P₁), psia 17.4 14.7 Outlet Pressure, psia 522441 Inlet Density, lb/ft³ 0.0671 0.0735 Outlet Density, lb/ft³ 0.8750.781 Weight Flow Ratio - (Inlet Cond)⁽³⁾ 1.00 1.00 Speed Ratio(N/N_(AIR))⁽⁴⁾ 0.99 1.00 # by adding known values for each gasconstituent multiplied by its percentage of the gas mixture.

[0075] After the exhaust E′ exits the output 44 of the turbine 40, theexhaust E′ is 55% CO₂ and 45% steam by weight. Some of CO₂ and the steamare produced within the combustor 30 and some enter the combustor 30through the oxidizer port 34 as part of the gas mixture C.

[0076] Preferably, the exhaust E′ is routed through the heat recoverysteam generator 50 so that heat can be removed from the exhaust E′ todrive a steam turbine and generator assembly 60 so that additional powercan be generated from the semi-closed combined cycle power system 100.However, utilization of the heat recovery steam generator 50 is notstrictly required for this power system 100. Rather, the turbine output44 could lead directly to the divider 110 so that a semi-closed simpleBrayton cycle variation on the power system 100 would be provided.

[0077] When the exhaust E′ exits the heat recovery steam generator 50 ithas a lower enthalpy but maintains its 55% wt CO₂ and 45% wt steamconstituent ratio. This exhaust E′ encounters the divider 110 where afirst portion thereof is directed to the return duct 120 for redirectionback to the compressor 20. A second portion of the exhaust E′ isdiverted by the diverter 110 to the separation duct 130. During steadystate operation, the amount of exhaust E′ directed to the separationduct 130 is precisely identical to the amount of CO₂ and steam which aregenerated by the combustion of methane with oxygen within the combustor.In essence, some of the CO₂ and water remain in the semi-closed powersystem 100 by passing along the return duct 120 and other portions ofthe exhaust E′ are removed from the power system 100 so that the amountof working fluid passing through the components making up the powersystem 100 remains constant.

[0078] Another serendipitous result of the semi-closed power system 100of this invention is that the exhaust E′ has a desired constituentmakeup which allows it to mimic air merely by having an appropriateamount of oxygen from the oxygen duct 150 added to a portion of theexhaust E′ within the return duct 120. Hence, the divider 110 need notdiscriminate between different constituent gases within the exhaust E′when dividing a portion of the exhaust E′ for passage along theseparation duct 130 and out of the system 100. Rather, it is merelynecessary that a homogeneous portion of the exhaust E′ which matches anamount of the exhaust E′ which was generated within the combustor asproducts of combustion of the methane and oxygen (approximately 15%) bediverted into the separation duct 130. This portion of the exhaust E′passing through the separation duct is thus approximately 55% CO₂ and45% water by weight.

[0079] The separation duct 130 directs this portion of the exhaust E′into the condenser 140. Because steam/water is ordinarily a liquid atstandard atmospheric conditions and CO₂ is typically a gas at standardatmospheric conditions, the condenser 140 can effectively and completelyseparate the steam/water constituent of the exhaust E′ from the CO₂constituent of the exhaust E′. Specifically, cooling water, cooling airor some other cooling medium is typically provided within the condenser140 to cool the exhaust E′ to below the condensation point for thesteam/water. The steam/water then condenses into a liquid and can beeasily removed from a bottom of the condenser 140 along an excess watercondensate outlet 142. Cooler portions of the condenser 140 which stillcontain gas lead to a gas outlet 144 which will be primarily CO₂, withsome water vapor remaining therein. Any water vapor in the CO₂ isremoved during its processing.

[0080] The excess water can be utilized in a variety of ways ordischarged into a surrounding environment. The excess CO₂ exiting thecondenser 140 along the gas outlet 144 is a substantially pure stream ofCO₂. While CO₂ can be exhausted into the atmosphere in an unregulatedfashion under current regulations, its separation into a substantiallypure stream at the gas outlet 144 allows for other uses for the CO₂rather than exhausting into the atmosphere.

[0081] Specifically, the CO₂ could be sold in the industrial gas market,it could be utilized in processes which benefit from use of CO₂, or itcan be pressurized into a liquid form for storage or disposal. Becauserelatively large quantities of CO₂ are produced by the combustion of themethane with oxygen, regardless of the beneficial uses provided for theCO₂, some CO₂ will remain which must be disposed of.

[0082] One known prior art technique is to pressurize the CO₂sufficiently so that it can be sequestered into underground depleted oilwells or other underground fissures, or directed into deep oceanlocations, or any other appropriate terrestrial formation. Suchpressurization only places a minimal efficiency penalty on thesemi-closed combined cycle power system 100 because substantially onlythe CO₂ need be pressurized. In contrast, the prior art combined cyclegas turbine 100 with an exhaust including nitrogen and CO₂ would incur asignificant efficiency penalty in separating the nitrogen and othergases from the CO₂ before pressurization of the remaining separated CO₂for sequestration. In essence, removal of oxygen from the air in an airseparation unit coupled to the oxygen duct 150 before combustion of thefuel with the gas mixture C, is more efficient than combusting the fuelin air and later separating the exhaust so that the CO₂ can beappropriately handled. Also, 100% of the CO₂ is removable, rather thanthe 85% to 90% removable with known prior art systems.

[0083] Typically, some water vapor may remain within the gas outlet 144.During the process of pressurizing the CO₂ within the gas outlet 144, aninitial stage of pressurization would typically cause remaining portionsof steam passing through the gas outlet 144 to be removed from the gasoutlet 144. Additionally, other gases which might remain within the gasoutlet 144 can be similarly removed. For instance, any excess oxygenwhich was not combusted within the combustor 30 can conceivably becaptured from the gas outlet 144 during the compression of the CO₂ inthat the oxygen would remain gaseous after the CO₂ has been liquefied.The oxygen could then be routed to the oxygen duct 150. Argon and othertrace gases could also be separated from the CO₂, for recovery and saleas by-products.

[0084] With particular reference to FIG. 4, details of the routing andtransformation of the working fluid within the semi-closed Brayton cycleof this invention are described. The working fluid of the semi-closedBrayton cycle includes two distinct mixtures at different portions ofthe cycle. A first oxidizer mixture of the working fluid includes oxygenor some other oxidizer and defines the working fluid of the cycleupstream of the combustor. A second exhaust mixture of the working fluidis located downstream of the combustor and reflects inclusion of thecombustion products created within the combustor.

[0085] While the oxidizer mixture and exhaust mixture forms of theworking fluid could have a variety of different constituent chemicalspecies, a preferred embodiment of this invention configures theoxidizer mixture and the exhaust mixture of the working fluid forcombustion of methane or natural gas (natural gas is largely methane)with oxygen at a near stoichiometric ratio. In such a configuration, theoxidizer mixture constituent chemical species includes gaseous molecularoxygen (O₂) as the only reactant constituent chemical species and amixture of carbon dioxide and water vapor as the diluent constituentelement species and as the product constituent chemical species. Whenmethane is combusted with oxygen the chemical equation for this reactionis as shown in equation 1.

CH₄+2O₂→CO₂+2H₂O  (equation 1)

[0086] This combustion reaction is a high energy exothermic reaction,resulting in release of substantial amounts of energy which in turnallow for a large amount of power generation by the semi-closed Braytoncycle power generation system of this invention. In fact, when methaneand oxygen are combusted at this stoichiometric ratio (equation 1) thecarbon dioxide and steam combustion products typically obtain atemperature of approximately 6,000° F. (depending on the temperature ofthe reactants and other combustion conditions).

[0087] The challenges associated with handling such high temperaturegases without damaging the combustor or the turbine downstream of thecombustor are not easily managed. To reduce this combustion productstemperature and confine the highest temperature combustion products toas small an area as possible, a non-reacting diluent is included as partof the oxidizer mixture.

[0088] In prior art Brayton cycle power generation systems, such anoxidizer mixture has merely been ambient air. Air has the requisiteamount of oxygen to form a reactant portion of the air oxidizer mixture.When the oxidizer mixture is air, the diluent portion is primarilygaseous nitrogen (N₂) with the nitrogen diluent constituting abouteighty percent of the oxidizer mixture. The diluent portion of theoxidizer mixture is not altered in the combustion reaction. Rather, itmerely passes through the combustor and helps regulate a temperature ofthe combustion products.

[0089] In essence, and as shown in FIG. 4, the reactant portion (oxygen)of the oxidizer mixture reacts with the fuel (methane) to produce thecombustion product portion (carbon dioxide and water) of the exhaustmixture form of the working fluid. The diluent portion (nitrogen) merelypasses through the combustor and is heated, forming the diluent portion(nitrogen) of the exhaust mixture form of the working fluid.Unfortunately, some of the nitrogen diluent portion does react with theoxygen at the high temperatures experienced within the combustor, suchthat nitrogen oxides (NO_(x)) are produced. Also, the nitrogen diluentportion of the exhaust mixture is difficult to separate from the carbondioxide constituent of the product portion (carbon dioxide and water) ofthe exhaust mixture. Hence, the nitrogen diluent interferes withcollection of the carbon dioxide in the exhaust mixture downstream ofthe combustor.

[0090] This invention uniquely replaces the nitrogen diluent portion inthe oxidizer mixture with a diluent portion comprised primarily ofcarbon dioxide and water. Hence, the oxidizer mixture is formed of thereactant portion (oxygen) and the diluent portion (carbon dioxide andwater). As with the prior art Brayton cycle, the reactant portion(oxygen) combusts with the fuel (methane) to form the product portion(carbon dioxide and water) of the exhaust mixture. The diluent portion(carbon dioxide and water) passes through the combustor substantiallyunaltered and forms the diluent portion (carbon dioxide and water) ofthe exhaust mixture.

[0091] Additionally, the ratio of carbon dioxide to water in the diluentportion of the oxidizer mixture is selected to match the ratio of carbondioxide to water in the product portion of the exhaust mixture. Hence,the exhaust mixture includes both a product portion (carbon dioxide andwater) and a diluent portion (carbon dioxide and water) which have thesame carbon dioxide to water ratios. The exhaust mixture is thus asingle homogeneous mixture of carbon dioxide and water having a carbondioxide to water ratio matching that which naturally occurs when oxygenand methane are combusted together at a stoichiometric ratio.

[0092] The exhaust mixture form of the working fluid is recirculatedaccording to the semi-closed Brayton cycle. Before this recirculation iscomplete, a portion of the exhaust mixture is diverted out of the cycleso that a total amount of carbon dioxide and water within the systemremains constant even though carbon dioxide and water are continuallybeing produced within the system. The recycled exhaust portion (carbondioxide and water) passes through a mixer where the reactant portion(oxygen) is added back to the recycled exhaust portion (carbon dioxideand water) to again form the oxidizer mixture form of the working fluid,including the reactant portion (oxygen) and the diluent portion (carbondioxide and water). This mixer of FIG. 4 is generally depicted in FIGS.2 and 3 where the oxygen supply duct 150 joins the return duct 120 butcould be a specific mixing structure located anywhere between theturbine/expander output and the compressor.

[0093] As specified in detail above, the specific constituentpercentages of the chemical species forming the reactant portion anddiluent portion of the oxidizer mixture and the product portion anddiluent portion of the exhaust mixture can vary to satisfy theparticular design criteria for any particular system. The two specificexamples identified above and the three gas composition formulationsidentified in example 2 are merely a few representative samples. Onesample overall formulation is shown below as equation 2.

6(CO₂+2H₂O)+2O₂+CH₄→7(CO₂+H₂O)  (equation 2)

[0094] In equation 2 the diluent portion (CO₂+2H₂O) is shown with aprevalence three times that of the reactant portion (O₂). Statedalternatively, seventy-five percent of the oxidizer mixture is diluentand twenty-five percent of the oxidizer mixture is oxygen. A greater orlesser ratio of diluent portion to reactant portion could be provided ifdesired. If a lesser diluent portion is provided, temperatures ofcombustion will typically be elevated. Conversely, if a greater amountof diluent is included, lesser temperatures would typically beexperienced. Certainly if other hydrocarbon fuels were utilized andcombusted with oxygen equations 1 and 2 could be appropriately modifiedto provide an appropriate balance of constituent chemical speciesforming the oxidizer mixture and the exhaust mixture forms of theworking fluid for the semi-closed Brayton cycle power generation systemof this invention.

[0095] The oxidizer mixture can be further modified according to thesteam injection variation of the semi-closed Brayton cycle shown indetail in FIG. 3. In such a steam injection variation, some of the waterin the exhaust mixture is not routed back to form a portion of theoxidizer mixture, but rather is routed directed to the combustor. Insuch a steam injection variation, the exhaust mixture would not bemodified from that described above. However, the oxidizer mixture wouldtend to have a greater ratio of CO₂ to water within the oxidizer mixtureafter diversion of a portion of the water/steam to the combustor. Whereit is desirable to have the oxidizer mixture mimic the characteristicsand performance of air, it would be generally desirable to increase aratio of reactant portion (oxygen) to diluent portion (carbon dioxideand water) to compensate for the lesser amount of water in the oxidizermixture where the steam injection variation of this invention isutilized.

[0096] In use and operation, and with particular reference to FIGS. 1and 2, details of the retrofitting of a combined cycle gas turbine powersystem 10 with components to make it into the semi-closed combined cyclepower system 100 of this invention are described. The semi-closed powersystem 100 of this invention can of course be constructed originallyfrom separate components as shown in FIG. 2. However, the power system100 utilizes each of the major components of a combined cycle gasturbine 10 so that retrofitting of the combined cycle gas turbine 10 canbe accomplished without replacement or modification of major components.

[0097] Specifically, the divider 110 is coupled to the existing exit 54of the heat recovery steam generator 50 and configured so that theseparation duct 130 and the condenser 140 extend from one portion of thedivider 110 and the return duct 120 extends from another portion of thedivider 110. The return duct 120 returns back to the inlet 22 of thecompressor 20. The oxygen duct 150 is positioned so that it can addoxygen into the return duct 120 and mix the oxygen with the exhaust E′to form the gas mixture C′ for use in the semi-closed power system 100.An air separation unit would typically also be provided, which would becoupled to the oxygen duct 150.

[0098] Hence, the combined cycle gas turbine 10 can be retrofitted intoa semi-closed combined cycle power system 100 with zero emissions (orvise versa for emergency operation) by merely adding the divider 110 andreturn duct 120 to route the exhaust E′ back to the compressor 20 andthe inclusion of a relatively straightforward separation duct 130,condenser 140 and air separation unit for oxygen production. Thisability to operate as a semi-closed cycle or an open cycle, in anemergency, provides the plant with a very high reliability factoragainst unscheduled shutdowns.

[0099] With particular reference to FIG. 3, the semi-closed Braytoncycle power system with steam injection 200 of this invention isdescribed in detail. The steam injection power system 200 begins witheach of the major components of the semi-closed power system 100 (FIG.2) described in detail above. However, for the steam injection powersystem 200, the semi-closed power system 100 is modified somewhat.Specifically, the heat recovery steam generator 50 is preferably notutilized to generate steam for a steam turbine and generator assembly60, but rather is utilized to generator steam for injection into thecombustor 30. Hence, the steam heating circuit 56, steam turbine andgenerator assembly 60 and working fluid pathway 62 are removed from thesemi-closed power system 100 (FIG. 2) when the system 100 is modified tooperate as the steam injection power system 200 (FIG. 3).

[0100] Additionally, the return duct 120 is modified to include apartial condenser 210 therein. The partial condenser 210 is preferablyinterposed within the return duct 120 so that all of the exhaust E′passes into the partial condenser 210. The partial condenser 210includes a cooling fluid circuit 212 which passes through the partialcondenser 210 in a heat exchange relationship so that a portion of thesteam within the exhaust E′ is caused to condense within the partialcondenser 210. The cooling fluid within the cooling fluid circuit 212could be any appropriate fluid, but would typically be water or possiblyair.

[0101] Lower surfaces of the partial condenser 210 include a condensateoutlet 214. The condensate outlet 214 leads to a water return line 216.This water return line 216 is routed back toward the heat recovery steamgenerator 50. The water within the water return line 216 is pumped up tocombustor inlet pressure with a pump 220.

[0102] A high pressure water line 217 exits the pump 220 and passesthrough the heat recovery steam generator as the steam generationcircuit 255. This steam generation circuit 255 outputs steam along steaminjection line 218. This steam is then injected into the combustor 30 atthe steam injection port 233.

[0103] The steam can in fact be introduced at a variety of locationsupstream from the turbine 40 or within the turbine 40. For instance, thesteam can be mixed with the compressed gas mixture exiting thecompressor 20 before this gas mixture enters the combustor 30 at theoxidizer port 34. The steam could also, or alternatively, be mixed withfuel entering the combustor 30 at the fuel port 32. It is also possiblethat the steam could enter the combustor 30 within a combustion chamberarea where maximum temperatures are achieved and cooling associated withthe steam injection would preserve hardware within the combustor 30. Thesteam can alternatively be introduced downstream from the combustor 30or at an intermediate stage within the turbine 40.

[0104] While the partial condenser 210 is preferably within the returnduct 120, it is also conceivable that the return duct 120 would be splitinto two pathways with one of the pathways having all or part of thesteam condensed therefrom and another of the pathways having no steamcondensed therefrom. These two pathways would then be rejoined togetherwith the same amount of water condensed from the exhaust E′ as in thepreferred embodiment identified above.

[0105] The steam injection power system 200 alters the make-up of theexhaust E′ due to the partial condensation of steam within the exhaustE′ while it flows through the return duct 120. Specifically, the exhaustE′ enters the partial condenser 210 preferably made up of 55% wt CO₂ and45% wt steam. A CO₂-enriched exhaust E″ exits the partial condenser 210within the return duct 120. The amount of steam depletion from theexhaust E″ depends on an amount of condensation occurring within thepartial condenser 210. The CO₂-enriched exhaust E″ then reaches theoxygen duct 150 where oxygen is added to the CO₂-enriched exhaust E″. Agas mixture C′ results which has a greater amount of CO₂ and oxygen anda lesser amount of steam when compared to the gas mixture C of theprevious embodiment (FIG. 2). An example of this gas mixture C′, whichis the preferred mixture for the steam injection power system 200, amongnumerous possible mixtures, is provided in example 2 which follows.

EXAMPLE 2

[0106] In this example, a little over half of the steam is condensedwithin the partial condenser 210. Specifically, the CO₂-enriched exhaustE″ exiting the partial condenser 210 includes 79% wt CO₂ and 21% wtsteam. Oxygen is supplied by the oxygen duct 150 at a ratio to theCO₂-enriched exhaust E″ to produce the gas mixture C′ including 63% wtCO₂, 17% wt steam and 20% wt oxygen. This gas mixture C′ has airmimicking attributes similar to those of the gas mixture C of theprevious embodiment (FIG. 2). Hence, the compressor 20 can compress thegas mixture C′ while operating within its design parameters for thecompression of air A (FIG. 1).

[0107] Note that the gas mixture C′ includes 20% wt oxygen which isapproximately the amount of oxygen within air A. This gas mixture C′ canbe utilized as the oxidizer within the combustor with an amount of fuelsimilar to that utilized when the combustor 30 is burning the fuel, suchas natural gas or methane, in air A (FIG. 1). In such a scenario, excessoxygen is typically present after full combustion of the fuel. Thisexcess oxygen within the working fluid exiting the combustor 30 wouldpass through the turbine 40 and be expanded along with otherconstituents of the exhaust E′.

[0108] The excess oxygen would then travel along with the exhaustthrough the divider 110 where a portion of the excess oxygen would passthrough the separation duct 130 and into the condenser 140. Such excessoxygen would exit the condenser 140 through the gas outlet 142. Theexcess oxygen could then be readily separated from the CO₂ and returnedto the oxygen duct 150 for reuse. The remaining oxygen within theexhaust E′ would pass through the divider 110 and on to the return duct120. It would then pass through the partial condenser 210 and be routedback to the compressor 30. Hence, in such an excess oxygen operationembodiment, a significant amount of oxygen would remain within theexhaust E′ (i.e. the exhaust E′ could have up to 10% wt oxygen) andCO₂-enriched exhaust E″ such that less oxygen would need to be suppliedfrom the oxygen duct 150 to create the gas mixture C′ at the desired 63%wt CO₂, 17% wt steam and 20% wt oxygen constituency.

[0109] Preferably however, when the gas mixture C′ is provided withoxygen as 20% of the gas mixture C′, a proportionately larger amount offuel, such as natural gas, is utilized within the combustor 30 so that astoichiometric or near stoichiometric ratio is still maintained betweenthe oxygen in the gas mixture C′ and the fuel. When such astoichiometric ratio is maintained, no oxygen remains within the exhaustE′. A greater amount of products of combustion would be provided perunit of gas mixture C′ than with the gas mixture C (FIG. 2).

[0110] This greater amount of combustion products would have itstemperature controlled by the injection of the steam into the combustor30 through the steam injection port 233. Hence, temperatures would notexceed the design parameters for the combustor 30 and the turbine 40.Significantly larger amounts of exhaust E′ exit the combustor and passthrough the turbine 40 per unit of gas mixture C′, such that the turbine40 outputs more power through power output 48. Because additional fuelis also utilized to burn with the additional amount of oxygen within thegas mixture C′, the overall efficiency of the steam injection powersystem 200, when compared to the semi-closed combined cycle power system100 is only slightly increased, while the power output from the steaminjection power system 200 is significantly increased.

[0111] While two steam injection embodiments have been disclosed whichutilize the 20% oxygen gas mixture C′, including an excess oxygenembodiment and a stoichiometric embodiment, it is understood thatnumerous different embodiments of the steam injection power system 200could be utilized having varying degrees of excess oxygen ranging fromthe excess oxygen present when air is utilized in a typical prior artgas turbine combustor 30 to the stoichiometric embodiment disclosedhereinabove. Each of these embodiments would include 20% oxygen withinthe gas mixture C′.

[0112] It is also conceivable that the gas mixture C′ could be furthermodified by condensing a greater or lesser amount of the exhaust E′and/or by adding more oxygen to the exhaust E″. Hence, gas mixturesother than the gas mixture C′ can result once the oxygen has been addedto the CO₂ enriched exhaust E″.

[0113] Table 2 provides a comparison of the twelve quantifiable gasparameters utilized in Table 1 above. In Table 2 characteristics of airare compared to three different gas mixtures including the gas mixture C(FIG. 2), the gas mixture C′ (FIG. 3) and an intermediate gas mixturewith gas constituencies within this intermediate gas mixture beingapproximately an average between the gas mixture C and the gas mixtureC′. This intermediate gas mixture illustrates how numerous different gasmixtures can be formulated which vary between the specific gas mixturesC and C′ disclosed in detail above, while still mimicking closely thecharacteristics of air. This intermediate gas mixture is considered tobe particularly desirable for use in compressors which are sensitive tosmall changes in gas characteristics, in that the intermediate gasmixture more closely mimics the characteristics of air. TABLE 2Parameter Gas Mixture¹ Gas Mixture² Gas Mixture³ Air Gas Constant-R,ft/° F. 56.6 51.5 45.7 53.3 Specific Heat - Btu/lb° F. 0.336 0.309 0.2790.250 Specific Heat Ratio (γ) 1.28 1.27 1.27 1.38 Typical Pressure Ratio30 30 30 30 Inlet Temperature (T₁), ° F. 200 180 160 80 OutletTemperature (T₂), ° F.⁽⁴⁾ 1058 979 934 1065 Inlet Pressure (P₁), psia17.4 16.1 14.9 14.7 Outlet Pressure, psia 522 483 447 441 Inlet Density,lb/ft³ 0.0671 0.0703 0.0757 0.0735 Outlet Density, lb/ft³ 0.875 0.9391.010 0.781 Weight Flow Ratio - (Inlet Cond)⁽⁵⁾ 1.00 1.00 1.00 1.00Speed Ratio, N/N_(AIR) ⁽⁶⁾ 0.99 0.916 0.849 1.00 # CO2, calculated byadding known values for each gas constituent multiplied by itspercentage of the gas mixture.

[0114] Specifically, the exhaust mixture of the turbine, assuming noexcess oxygen, would typically have between about fifty percent andsixty percent CO₂ by weight and between forty percent and fifty percentH₂O by weight. Most preferably, the exhaust mixture would be aboutfifty-five percent CO₂ and about forty-five percent H₂O. If the systemoperates with excess oxygen, these percentages would proportionatelydecrease.

[0115] The oxidizer mixture could be varied widely to accommodate directsteam injection (hence reducing a water vapor portion of the oxidizermixture) and to provide excess oxygen or to provide better matching ofair characteristics for optimal compressor performance. Overall byweight, the carbon dioxide could vary from about thirty percent or fortypercent to about seventy percent or eighty percent. The water vaporcould vary from about ten percent or twenty percent to about forty-fivepercent or fifty percent. The oxygen could vary from about ten percentor fifteen percent to about twenty-five percent or thirty percent.Narrower constituent ranges of about five percent or ten percentsurrounding the optimal formulations cited as examples would tend toprovide air mimicking characteristics and performance closer to theoptimal formulations.

[0116] In use and operation, and with particular reference to FIGS. 1-3,details of the retrofitting of a combined cycle gas turbine power system10 with components to make it into the semi-closed steam injection powersystem 200 of this invention are described. The semi-closed steaminjection power system 200 of this invention can of course beconstructed originally from separate components as shown in FIG. 3.However, the steam injection power system 200 utilizes each of the majorcomponents of a combined cycle gas turbine 10 (other than the steamturbine and generator assembly 60), so that retrofitting of the combinedcycle gas turbine 10 can be accomplished in a straightforward manner.

[0117] Specifically, the divider 110 is coupled to the existing exit 54of the heat recovery steam generator 50 and configured so that theseparation duct 130 and the condenser 140 extend from one portion of thedivider 110 and the return duct 120 extends from another portion of thedivider 110. The return duct 120 includes a partial condenser 210therein. The partial condenser 210 receives cooling fluid and causes aportion of the steam passing through the return duct 120 to be condensedinto water which exits the partial condenser 210 along the condensateoutlet 214 and water return line 216.

[0118] A pump 220 is provided so that the water can be pumped throughthe heat recovery steam generator 50. The steam turbine and generatorassembly 60 of the combined cycle gas turbine power system 10 areremoved and the steam heating circuit 56 within the heat recovery steamgenerator 50 is now utilized to generate steam with the water from thepump 220 and the partial condenser 210. Steam is delivered from thesteam generation circuit 255 within the heat recovery steam generator 50and directed to the steam injection port 233 of the combustor 30. Hence,the combustor 30 is modified appropriately to include this steaminjection port 233. The return duct 120 returns the CO₂-enriched exhaustE″ back to the inlet 22 of the compressor 20. The oxygen duct 150 addsoxygen into the return duct 120 to mix with the exhaust E″ to form thegas mixture C′. An air separation unit would typically also be provided,which would be coupled to the oxygen duct 150.

[0119] If the combined cycle gas turbine power system 10 is alreadyconfigured to be a steam injection power system, the combustor 30 willalready include the appropriate steam injection port 233. In such aconfiguration, removal of the steam turbine and generator assembly 60would not be required. Rather, the water return line 216 from thepartial condenser 210 would merely be coupled to the feed water inlet ofthe combined cycle gas turbine power system 10. Hence, either a steaminjection open Brayton cycle gas turbine or the combined cycle gasturbine 10 can be retrofitted into a semi-closed steam injection powersystem 200 with zero emissions.

[0120] With particular reference to FIG. 5, details of an air startupmode semi-closed Brayton cycle power system 259 is shown. Thesemi-closed Brayton cycle power systems 100, 200 of FIGS. 2 and 3 depicta steady state operating mode for these systems after the diluentmixture of the working fluid is in the form of carbon dioxide and water.While it would be possible to provide a carbon dioxide and water vapordiluent startup chamber so that the proper carbon dioxide and watervapor diluent would be provided immediately upon startup of the system100, 200, system 259 of FIG. 5 depicts an alternative where the system259 initially starts up with air forming the oxidizer mixture forstartup convenience.

[0121] Specifically, a portion of the return duct upstream of the inlet22 of the compressor 20 includes a valve preferably in the form of aseries of inlet louvers 260. When these inlet louvers 260 are open, theair can pass directly into the return duct 120 upstream of thecompressor 20.

[0122] A second valve, preferably in the form of a set of early outletlouvers 270, could be provided downstream of the output 44 of theturbine 40 so that the exhaust could be released from the system 259during startup. A third late outlet louver set 280 could be provideddownstream of the heat recovery steam generator 50. During startup, theinlet louvers 260 would be opened and the early outlet louvers 270 wouldbe opened. The compressor 20, combustor 30 and turbine 40 would then befired up utilizing known gas turbine startup techniques. Once thecompressor 20, combustor 30 and turbine 40 are all operational, theearly outlet louvers 270 can be closed and the late outlet louvers 280opened. In this way, the high temperature exhaust products of combustionare routed through the heat recovery steam generator 50 and thebottoming cycle including the steam turbine generator 60 (FIG. 2) can befired up with the heat from the heat recovery steam generator 50.Alternatively, the heat recovery steam generator 50 can commencepreheating of water/steam for injection into the combustor 30 accordingto the steam injection system 200 (FIG. 3).

[0123] Finally, the air separation unit or other gaseous oxygen supplysystem can commence delivery of oxygen through the oxygen duct 150 intothe return line 120. The late outlet louvers 280 can be closed and theinlet louvers 260 closed so that the recirculating exhaust products ofcombustion and the oxygen in the oxygen duct 150 can provide theoxidizer upstream of the compressor 20 for continuing operation of thesystem 250 as a semi-closed Brayton cycle in its optimum form as shownin FIGS. 2 and 3. If necessary, additional louvers or other valves canalso be located in the return duct 120 and/or the oxygen duct 150 towhich would close when the air inlet louvers 260 are opened and openwhen the air inlet louvers 260 are closed. While nitrogen in the airwould not immediately leave the system 259 it would gradually bediverted into the gas outlet 142 of the condenser 140 until no nitrogenremains.

[0124] Another advantage of the startup mode variation on thesemi-closed Brayton cycle power system 259 is that the system 259 canalternate between operation with air and operation in a semi-closed modewith recycled products of combustion and oxygen entering the compressor20. Such flexibility allows the return duct 120 and/or the oxygen duct150 and associated oxygen supply equipment to be taken off-line ifnecessary. For instance, maintenance or a failure in the semi-closedBrayton cycle subsystems could result in conversion to operation withair as the oxidizer, without influencing power output from the system259 and without requiring that the power system 259 be taken “off-line.”Additionally, should the power system 259 be located in a region withrelatively lower environmental sensitivity to air pollution and if aneed for increased power output from the overall power system 259 isoccasionally required, the system could shift to air operation forenhanced power output in a polluting mode when acceptable.

[0125] With particular reference to FIGS. 6-10, five different advancedvariations on the semi-closed Brayton cycle power plant are described.These advanced variations are briefly described overall including themany various subsystems which are identical to the systems 100, 200 ofFIGS. 2 and 3 above. Additional information is provided regarding howeach of these advanced systems of FIGS. 6-10 vary from the systems 100,200 of FIGS. 2 and 3 above.

[0126] With particular reference to FIG. 6, details of the semi-closedBrayton cycle power system 300 featuring use of an ion transfer membraneair separation unit 330 are described. Cryogenic air separation is thecurrent standard method for producing large daily tonnages of relativelypure oxygen that are needed for semi-closed Brayton cycle power systems100, 200 (FIGS. 2 and 3). The cryogenic process represents asubstantially mature technology that has advanced far along the classic“learning curve.” The largest single train air separation units can nowproduct approximately 3,500 tons per day (TPD) of oxygen, a quantitysufficient to support a power plant having roughly 200 Mwe output.Larger single train units (>5,000 TPD) appear possible and do offer somemodest improvement in economy-of-scale. Continued, although incremental,improvements in the technology, performance, and cost of some majorcomponents of air separation units such as specialty heat exchangers andcompressors can also be expected. Some cost improvements will continuein the cryogenic air separation industry but the trend lines for thelearning curve and economy-of-scale benefits predict smaller percentageimprovements in the future. Thus, a major reduction in oxygen costs willrequire the development of an advanced air separation technology. Oneadvanced air separation technology is based on ion transport membrane(ITMs). ITMs are solid materials that produce oxygen by the passage ofoxygen ions through ceramic materials containing selected inorganicoxide materials. They operate at high temperatures in direct contrast tothe cryogenic technology, generally over 900° F. Oxygen molecules areconverted to oxygen ions at the surface of the membrane and aretransported through the membrane by an applied voltage or a pressuredifferential, reforming to oxygen molecules at the other membranesurface.

[0127] Government and industry are currently developing the ITM airseparation technology. This technology has the potential to reduce theenergy consumed to produce oxygen by fifty percent and the capital costby twenty-five percent over conventional cryogenic air fractionationprocesses. An important benefit of the ITM technology for large-scaleoxygen production is the ability to integrate it with gas or advancedsteam turbine systems, as illustrated in FIG. 6, to provide pressurizedair and heat to operate the system.

[0128] The semi-closed Brayton cycle gas turbine system 800 (FIG. 6)described below recirculates a mixture of carbon dioxide (CO₂) and steam(H₂O), called Cgas, at mixture ratios that approximate air properties.By using such gaseous mixtures, existing gas turbine compressors andturbines can be used with no significant design modification. Acomparison of these gas properties with those of air is shown in Table 1above.

[0129]FIG. 6 illustrates this concept with a non-polluting efficientelectrical energy power plant 300, comprising an air induction subsystem310, air preheating subsystem 320, an ion membrane air separationsubsystem 330, a gas compression subsystem 340, a gas generationsubsystem 350, a gas turbine drive subsystem 360, an electric energygeneration subsystem 370, a heat recovery subsystem 380, an H₂O/CO₂separation subsystem 390 and a gas separation/water management subsystem395.

[0130] The air induction subsystem 310 feeds and controls the air supplyto generate the oxygen needed by the plant 300. The air is heated toapproximately 900° F. with low energy steam/CO₂ mixture, from theexhaust of the gas turbine drive subsystem 360 in the heat exchanger ofsubsystem 320. The warm air then enters the ion membrane plant 330 whereoxygen/nitrogen separation takes place. The nitrogen is collected,cooled, and sold as a byproduct or discharged back to the atmosphere.The nearly pure oxygen is collected, cooled and directed to the gascompression subsystem 340 for compression along with the carbon dioxideand water vapor from the return duct downstream of the turbine output.

[0131] Before the nitrogen is released or collected, its excess heat ispreferably used to preheat incoming air and to preheat water between thepartial condenser 395 and the combustor 350. Similarly, excess heat inthe oxygen is used to preheat incoming air and water downstream of thepartial condenser 395.

[0132] The compressed gas exiting subsystem 340 is then directed to thegas generation subsystem 350. In the gas generation subsystem 350 fueland oxidizer from the compressor 340, and optionally recirculatingheated water (steam) from subsystem 380 are mixed and combust at a nearstoichiometric mixture ratio to generate the drive gas of approximatelyfifty-five percent CO₂ and forty-five percent H₂O for the gas turbinesubsystem 360. The turbine of subsystem 360 then drives the generator ofsubsystem 370 to produce electricity. The gas exiting the turbine fromsubsystem 360 then enters the heat recovery steam generation subsystem380 where recirculating water is heated for injection into the gasgeneration subsystem 350 and for preheating the air in subsystem 320.The turbine exhaust gases of fifty-five percent CO₂ and forty-fivepercent H₂O exiting the heat recovery subsystem 380 are directed to thecondensers of subsystem 390 and 395.

[0133] The excess gas of CO₂/H₂O enters the condenser of subsystem 390where the steam condenses into water and separates from the CO₂. The CO₂is pumped from the condenser of subsystem 390 can be used as makeupwater for other subsystems. The remainder of the gas from subsystem 380enters the partial condenser of subsystem 395 where a portion of thesteam is condensed and the remaining portion of the steam and CO₂ isreturned to the compressor of the subsystem 340 where it is mixed withoxygen coming from subsystem 330. The condensed water from subsystem 395is directed to the heat recovery subsystem 380 where heat from theturbine exhaust of subsystem 360 is recovered and the recirculatedheated water (steam) is injected into the gas generation subsystem 350.The cooling water for subsystem 395 rejects the heat absorbed from thesteam/CO₂ gases.

[0134] The principle features of power plant 300 include the integrationof the ion transfer membrane (ITM) air separation unit (ASU) with a highefficiency gas turbine subsystem 360 to produce low cost electricity andto permit CO₂ separation, conditioning and preparation for sequestrationinto underground or undersea sequestration sites with the lowest costenergy penalty possible. The recirculation of steam and CO₂ at mixtureratios that provide air-like characteristics, permit the use of existinggas turbines and thus eliminate the cost for new hardware development.Also, the condensation of some of the recirculating steam into water andreheating it with the turbine exhaust gases for injection as steam intothe turbine has the beneficial features of: (1) reducing compressorwork; (2) increasing turbine power; and (3) eliminating the need for abottoming cycle steam turbine, without sacrificing efficiency. The laterbenefit reduces power plant cost and complexity, and increases powerplant reliability due to fewer number of subsystems. All these featuresare achieved with zero emissions and permit power generation suing themost abundant energy available, fossil fuels, without the harmfuldischarge of pollutants or greenhouse gases to the atmosphere.

[0135] Integration of the system 300 with the ion transfer membrane airseparation unit 330 enhances the efficiency of operation of the iontransfer membrane air separation unit 330. Specifically, because excessheat from the output of the turbine 360 is used to heat the air in thepreheating subsystem 320, a separate heater for the ion transfermembrane air separation unit 330 is not required. In essence, waste heatfrom the turbine 360 is beneficially used to provide the necessary heatfor operation of the ion transfer membrane air separation unit 330. Theion transfer membrane air separation unit 330 is additionally integratedinto the overall system 300 by utilizing the excess heat remaining inthe oxygen and nitrogen exiting the ion transfer membrane to preheat airentering ion transfer membrane air separation unit 330 and to alsopreheat water which requires heating to boil into steam before injectioninto the combustor 350.

[0136] With particular reference to FIG. 7, a semi-closed Braytoncombined cycle power plant 400 with an ion transfer membrane oxygenplant variation on the system 300 of FIG. 8 is described whichsubstitutes steam injection as shown in FIGS. 3 and 5 with a heatrecovery steam generator 480 feeding a steam turbine and generator 410in a bottoming cycle similar to that provided in the system 100 of FIG.2.

[0137] For simplicity, only features of the system 400 of FIG. 7 whichare unique from the system 300 of FIG. 6 are described in detail.Particularly, a bottoming cycle similar to that described above withFIG. 2 is fed with steam from a heat recovery steam generator 480. Thissteam is passed to a steam turbine and generator 410, preferably forthis embodiment of a typical Rankine cycle variety. High temperaturesteam is removed either before the steam enters the steam turbine andgenerator 410 or steam at an intermediate stage within the turbine, sothat high temperature steam (approximately 1,000° F.) is fed to the iontransfer membrane air separation unit to preheat incoming air at theheat exchanger 420. This diverted is then returned to the steam turbineand generator 410 to do further work within the steam turbine. The steamexiting the steam turbine and generator 410 is then typically routedthrough a condenser 430 and then through a condensate pump 440.

[0138] Before the water is fed back to the heat recovery steam generator480, the water is preferably preheated by passing through heatexchangers 450 and 460 which preheat the water with heat from thenitrogen and oxygen exiting the ion transfer membrane air separationunit of the system 400. The preheated water/steam is then fed to theheat recovery steam generator 480. If the water is still a liquid itwould typically be passed through feed water pump 470. Alternatively,the condensate pump 440 could pressurize the water to the extentnecessary for operation within the steam turbine and generator 410. Theremainder of the system 400 operates in a manner similar to thatdisclosed with regard to the system 300 of FIG. 8. However, because theheat recovery steam generator 480 utilizes the excess heat from the gasturbine exhaust, steam is not injected into the combustor as is the casewith the system 300 of FIG. 6.

[0139] With particular reference to FIG. 8, a semi-closed Braytoncombined cycle power plant 500 is disclosed which has both a bottomingcycle fed by a heat recovery steam generator and steam injection intothe combustor. With the system 500, the cycle can operate with steaminjection into the combustor of the gas turbine or with steam beinggenerated for a bottoming cycle steam turbine and generator similar tothat shown in FIG. 7, or both to varying different degrees. The system500 of FIG. 8 thus provides both the benefits of the steam injectionsystem 300 of FIG. 6 and the combined cycle variation 400 of FIG. 7.

[0140] With particular reference to FIG. 9, the semi-closed Braytoncombined cycle power system 600 is disclosed similar to the system 500of FIG. 8 except with supplementary heating within the bottoming cycleof the combined cycle. Only features which differ from the previoussystems 300, 400, 500 of FIGS. 6-8 are described in detail. Rather thanmerely providing heating within the heat recovery steam generator 480 ofthe system 400 of FIG. 7, the system 600 includes a combustor/gasgenerator 610 between the heat recovery steam generator and the turbineand generator 630. Preferably, the turbine and generator 630 areconfigured as a high temperature steam turbine and generator. Thecombustor 610 can be similar to the combustor of the gas turbine (i.e.combustor 30 of FIG. 2). Preferably, however, the diluent is not mixedwith the oxygen entering the combustor/gas generator 610, and is in theform of substantially pure water, rather than a mixture of water andcarbon dioxide. Such a gas generator could be similar to that disclosedin U.S. Pat. Nos. 6,206,684; 5,969,786 and 5,956,937, each of whichpatents are incorporated herein by reference.

[0141] As with the system 500 of FIG. 8, some steam is diverted topreheat air entering the ion transfer membrane air separation unit. Aportion of the oxygen from the ion transfer membrane air separation unitis routed to the combustor/gas generator 610 through an oxygenconditioning system 620. This oxygen conditioning system 620 receiveslow pressure oxygen which would typically be liquefied, pumped to a highpressure and reheated to ambient temperature so that the oxygen could bepressurized as efficiently as possible before delivery to thecombustor/gas generator 610.

[0142] The exhaust of the high temperature steam turbine engine 630would be a mixture of steam and carbon dioxide. Condenser 640 wouldseparate the steam from the carbon dioxide. The carbon dioxide could becollected at 650 for storage, industrial use or injection intoterrestrial formations such as partially or completely depleted oilwells, deep ocean locations or other sequestration sites. Water exitingthe condenser 640 would be routed in a fashion similar to the watercondensate of the system 500 of FIG. 8. While this system 600 is shownwithout steam injection into the combustor of the gas turbine, featuresof the system 300 of FIG. 6 could be added to the system 600 in afashion similar to that depicted with the system 500 of FIG. 8.

[0143] With particular reference to FIG. 10, details of a semi-closedBrayton combined cycle power plant 700 integrated with an ion transfermembrane air separation unit and featuring intercooling, reheating andsupplementary heating is described. The semi-closed combined cycle 700with supplementary heating, compressor interstage cooling and turbineinterstage reheating described below recirculates a mixture of carbondioxide (CO₂) and steam (H₂O), called Cgas, at mixture ratios thatapproximate air properties. By using such gaseous mixtures, existing gasturbine compressors and turbines can be used with no significant designmodifications. A comparison of these gas properties with those of air isshown in Table 1. Also, the addition of a steam turbine to recovery theresidual exhaust heat of the gas turbine and the gas generator to boostthe steam to maximum operating temperatures, increases the power andefficiency of the combined-cycle to its maximum capabilities.

[0144]FIG. 10 illustrates this concept with a non-polluting efficientelectrical energy power plant 700, comprising an air induction subsystem710, air preheating subsystem 720, an ion membrane air separationsubsystem 730, a low pressure gas compression subsystem 740, a highpressure gas compression system 750, with gas intercooling betweenstages, a combustor gas generation subsystem 760, a high pressure gasturbine drive subsystem 770, a gas reheating combustor 780, a lowpressure gas drive subsystem 790, an electric energy generationsubsystem 810, a heat recovery subsystem 820, a H₂O/CO₂ separationsubsystem 830, a gas separation/water management subsystem 840, a heatrecovery steam generator subsystem 850, a supplementary heating gasgenerator subsystem 860, a high pressure, high temperature steam turbinedrive subsystem 870, and a condenser H₂O/CO₂ water management subsystem880.

[0145] The air induction subsystem 710 feeds and controls the air supplyto generate the oxygen needed by the plant 700. The air is heated toapproximately 900° F. with low energy steam/CO₂ mixture, preferably fromsteam of subsystem 870, in the heat exchanger of subsystem 720. The warmair then enters the ion membrane plant 730 where oxygen/nitrogenseparation takes place. The nitrogen is collected, cooled, and sold as abyproduct or discharged back to the atmosphere. The nearly pure oxygenis collected, cooled and directed along with carbon dioxide and watervapor to the lower pressure gas compression subsystem 740 and highpressure gas compression subsystem 750. The compressed gas exitingsubsystem 750 is then directed to the gas generation subsystem 760.

[0146] In the combustor gas generation subsystem 760, fuel andrecirculating heated water (steam) from the subsystem 820 are mixed andcombust near stoichiometric mixture ratio to generate the drive gas ofapproximately fifty-five percent CO₂ and forty-five percent H₂O for thehigh pressure gas turbine subsystem 770. The turbine exhaust gas fromsubsystem 770 then enters the combustor subsystem 780 where additionalfuel is added to the oxidizer rich gas to reheat the gas prior toentering the low pressure turbine of subsystem 790. The turbines ofsubsystem 780 and 790 then drive the generator of subsystem 810 toproduce electricity and subsystem compressors 740 and 750.

[0147] The gas exiting the turbine from subsystem 790 then enters theheat recovery steam generation subsystem 820 where recirculating wateris heated for injection into the gas generation subsystem 760. Theturbine exhaust gases of fifty-five percent CO₂ and forty-five percentH₂O exiting the heat recovery subsystem 820 are directed to thecondensers of subsystems 830 and 840. The excess gas of CO₂/H₂O entersthe condenser of subsystem 830 where the steam condenses into water andseparates from the CO₂. The CO₂ is pumped from the condenser ofsubsystem 830 and processed for sale as a byproduct, discharged to theatmosphere or prepared for sequestration into underground or deep oceandisposal sites. The excess water from subsystem 830 can be used asmakeup water for other subsystems. The remainder of the gas fromsubsystem 820 enters the condenser of subsystem 840 where a portion ofthe steam is condensed and the remaining portion of steam and CO₂ isreturned to the compressor of subsystem 740 where it is mixed withoxygen coming from subsystem 730.

[0148] The condensed water from subsystem 840 is directed to the heatrecovery subsystem 820 where heat from the turbine exhaust of subsystem790 is recovered and the recirculated heated water (steam) is injectedinto the gas generation subsystem 760. Exhaust gases from the turbine ofsubsystem 790 are also used to heat the steam in the heat recovery steamgenerator (HRSG) of subsystem 850.

[0149] The gas generator of subsystem 860 boosts the steam temperatureto its maximum value and drives the steam turbine and generator ofsubsystem 870 is a manner similar to that described above with system600 of FIG. 9. The exhaust steam from subsystem 870 is condensed andrecirculated by the condensate and feed water pumps of the watermanagement system 880. The condensed water from the water managementsystem 880 is also used to cool the oxygen and nitrogen gases, generatedby the ion transfer membranes (ITM) of the air separation unit (ASU) ofsubsystem 730, to near ambient temperatures, in turn preheating thewater before return to the gas generator 860.

[0150] With particular reference to FIG. 11, details of a uniqueretrofit of a gas turbine to enhance power output from the gas turbine,generate CO₂ efficiently with the gas turbine, and not increase theenvironmental pollutants generated by the gas turbine is described. Thisgas turbine combined cycle system 900 is similar to the gas turbine 10described in FIG. 1 except as specifically described herein.

[0151] Specifically, the heat recovery steam generator 910 feeds hightemperature steam to a gas generator 920 similar to the gas generator860 of the system 700 described in FIG. 10. This gas generatoradditionally is fed methane or another hydrocarbon fuel along withoxygen from an air separation plant 930 or some other oxygen source. Thegas generator 920 produces high temperature and high pressure combustionproducts including steam and carbon dioxide along with the steam fromthe heat recovery steam generator 910 to a steam turbine and generator940.

[0152] After power is produced by the steam turbine and generator 940,the combustion products including steam and carbon dioxide are passed onto a condenser 950. The condenser removes excess water and recycleswater to the heat recovery steam generator 910. The condenser alsoseparates gaseous CO₂ for collection and further industrial use or safedisposal. This system 900 causes a standard gas turbine combined cycle10 to generate a pure stream of carbon dioxide and generate additionalpower from the steam turbine and generator 940 without altering any ofthe subsystems in the existing power gas turbine combined cycle 10 inany fashion. This system 900 does require the addition of the airseparation plant 930 or other oxygen supply along with the associatedbottoming cycle equipment.

[0153] This disclosure is provided to reveal a preferred embodiment ofthe invention and a best mode for practicing the invention. Having thusdescribed the invention in this way, it should be apparent that variousdifferent modifications can be made to the preferred embodiment withoutdeparting from the scope and spirit of this disclosure. When structuresare identified as a means to perform a function, the identification isintended to include all structures which can perform the functionspecified. No claim language other than that explicitly accompanied bythe word “means” should be construed as intending to invokeinterpretation of that language as “means plus function” languageaccording to section 112 of Title 35 of the United States code.

What is claimed is:
 1. A semi-closed Brayton cycle power generationsystem, comprising in combination: a gas compressor having an inlet andan outlet; a combustor downstream of said compressor, said combustorhaving a fuel port coupled to a source of fuel, an oxidizer port coupledto said compressor outlet and an outlet port for combustion productsresulting from combustion of fuel from said source of fuel with oxidizerfrom said oxidizer port; a turbine downstream of said combustor, saidturbine having an input coupled to said combustor outlet port, an outputfor the combustion products entering said turbine at said input, and apower output; a return duct downstream of said turbine, said return ductreceiving at least a portion of the combustion products passing throughsaid output of said turbine and extending to said inlet of saidcompressor; a gaseous oxygen duct coupled to a source of oxygen andcoupled to said return duct in a manner adding oxygen to the combustionproducts within said return duct; and said gaseous oxygen duct locatedupstream of said gas compressor, such that at least a portion of theoxygen from said gaseous oxygen duct enters said compressor inlet alongwith the combustion products.
 2. The system of claim 1 wherein an airinlet duct is coupled to said return duct at a location upstream of saidgas compressor.
 3. The system of claim 2 wherein said air inlet duct hasa valve thereon said valve capable of selectively opening and closingsaid air inlet duct.
 4. The system of claim 3 wherein an outlet duct islocated downstream of said turbine output, said outlet duct leading outof said system, such that when said air inlet duct is open and saidoutlet duct is open, said system can operate as an open Brayton cyclepower generation system.
 5. The system of claim 4 wherein a valve islocated on said outlet duct.
 6. The system of claim 5 wherein a heatrecovery steam generator is located downstream of said turbine output,said heat recovery steam generator transferring heat out of thecombustion products exiting said turbine output, said outlet ductlocated upstream of said heat recovery steam generator and downstream ofsaid turbine output.
 7. The system of claim 5 wherein a heat recoverysteam generator is located downstream of said turbine output, said heatrecovery steam generator transferring heat out of the combustionproducts exiting said turbine output, said outlet duct locateddownstream of said heat recovery steam generator.
 8. A method forstarting a semi-closed Brayton cycle power system including the stepsof: providing a semi-closed Brayton cycle power generation systemincluding: a gas compressor having an inlet and an outlet; a combustordownstream of the compressor, the combustor having a fuel port coupledto a source of fuel, an oxidizer port coupled to the compressor outletand an outlet port for combustion products resulting from combustion offuel from the source of fuel with oxidizer from the oxidizer port; aturbine downstream of the combustor, the turbine having an input coupledto the combustor outlet port, an output for the combustion productsentering the turbine at the input, and a power output; a return ductdownstream of the turbine, the return duct receiving at least a portionof the combustion products passing through the output of the turbine andextending to the inlet of the compressor; a gaseous oxygen duct coupledto a source of oxygen and coupled to the return duct in a manner addingoxygen to the combustion products within the return duct; the gaseousoxygen duct located upstream of the gas compressor, such that at least aportion of the oxygen from the gaseous oxygen duct enters the compressorinlet along with the combustion products; and wherein an air inlet ductis coupled to the return duct at a location upstream of the gascompressor; opening the air inlet duct so that air can pass into thereturn duct upstream of the compressor inlet; keeping the oxygen ductinitially closed; starting the combustor, the turbine and the compressorof the semi-closed Brayton cycle power generation system; opening theoxygen duct; and closing the air inlet duct.
 9. The method of claim 8including the further step of removing a nitrogen constituent of airentering the system at the air inlet duct during said opening step byincluding a divider downstream of the turbine output, the dividerleading a portion of an exhaust from the turbine to a separation ductleading away from the inlet of the compressor, and allowing the nitrogento gradually separate out of the system through the separation duct. 10.The method of claim 8 including the further step of providing an outletduct, opening the outlet duct before said starting step; and closing theoutlet duct after said starting step.
 11. A semi-closed Brayton cyclepower generation system, comprising in combination: a gas compressorhaving an inlet and an outlet; a combustor downstream of saidcompressor, said combustor having a fuel port coupled to a source offuel, an oxidizer port coupled to said compressor outlet and an outletport for combustion products resulting from combustion of fuel from saidsource of fuel with oxidizer from said oxidizer port; a turbinedownstream of said combustor, said turbine having an input coupled tosaid combustor outlet port, an output for the combustion productsentering said turbine at said input, and a power output; a return ductdownstream of said turbine, said return duct receiving at least aportion of the combustion products passing through said output of saidturbine and extending to said inlet of said compressor; a gaseous oxygenduct coupled to a source of oxygen and coupled to said return duct in amanner adding oxygen to the combustion products within said return duct;said gaseous oxygen duct located upstream of said gas compressor, suchthat at least a portion of the oxygen from said gaseous oxygen ductenters said compressor inlet along with the combustion products; saidsource of oxygen including an ion transfer membrane between an air inletand an oxygen outlet, said oxygen outlet coupled to said gaseous oxygenduct; and an air heater between said air inlet and said membrane, saidheater transferring heat from the combustion products downstream of saidcombustor into the air entering said air inlet.
 12. The system of claim11 wherein a diversion line is located downstream of said turbineoutput, said diversion line directing combustion products to said heaterbetween said air inlet and said membrane.
 13. The system of claim 11wherein a heat recovery steam generator is located downstream of saidturbine output, said heat recovery steam generator removing heat fromthe combustion products exiting said turbine output, said heat recoverysteam generator heating water upstream of a steam turbine; and adiversion line diverting water between said heat recovery steamgenerator and said steam turbine to said heater between said air inletand said membrane before said diversion line returns to said steamturbine.
 14. The system of claim 11 wherein a heat recovery steamgenerator is located downstream of said turbine output, said heatrecovery steam generator removing heat from the combustion productsexiting said turbine output, said heat recovery steam generator heatingwater upstream of a steam turbine; and a diversion line extending from alocation midway between an inlet of said steam turbine and an outlet ofsaid steam turbine, said diversion line routed to said air heaterbetween said air inlet and said membrane before said diversion linereturns to said steam turbine.
 15. The system of claim 11 wherein apartial condenser is located downstream of said turbine, said partialcondenser including a condensate line for removal of condensed water outof said partial condenser, said condensation line routed to saidcombustor along a path bypassing said compressor, said path including awater preheater there along, said water preheater receiving heat fromnitrogen discharged from said source of oxygen.
 16. The system of claim15 wherein said condensate line includes a second preheater receivingheat from oxygen discharged from said oxygen outlet of said source ofoxygen between said membrane and said gaseous oxygen duct.
 17. Thesystem of claim 11 wherein a heat recovery steam generator is locateddownstream of said turbine output, said heat recovery steam generatorremoving heat from combustion products exiting said turbine output andadding heat to a bottoming cycle working fluid upstream of a bottomingcycle turbine, a diversion line downstream of said bottoming cycleturbine including a preheater thereon, said preheater heating thebottoming cycle working fluid with heat from hot gases exiting saidsource of oxygen before said bottoming cycle working fluid is routedback to said heat recovery steam generator.
 18. The system of claim 11including a heat recovery steam generator downstream of said turbineoutput, said heat recovery steam generator transferring heat from thecombustion products exiting said turbine output to water in a bottomingcycle coupled to said heat recovery steam generator, said bottomingcycle including a gas generator having an oxygen inlet, a hydrocarbonfuel inlet and a water inlet, said water inlet coupled to said heatrecovery steam generator to receive water from said heat recovery steamgenerator, said gas generator combusting said hydrocarbon fuel with saidoxygen to produce combustion products including carbon dioxide andwater, said bottoming cycle including an expander downstream of said gasgenerator and a condenser downstream of said expander, said condenserseparating at least a portion of the water in the combustion productsfrom a portion of the carbon dioxide in the combustion products, atleast a portion of the water in the combustion products recirculated tosaid heat recovery steam generator.
 19. The system of claim 18 wherein acompressor is located downstream of a gas outlet of said condenser, saidcompressor configured to compress gases including CO₂ passingtherethrough to sufficient pressure to allow injection of the gasesincluding CO₂ into a terrestrial formation for elimination of the CO₂from atmospheric release.
 20. The system of claim 19 wherein said oxygeninlet of said gas generator is coupled to said oxygen outlet of saidsource of oxygen and wherein said source of oxygen includes a hotnitrogen outlet, and a heat exchanger between said hot nitrogen outletand a condensate outlet of said condenser of said bottoming cycle, suchthat condensate from said condenser including water is preheated withheat from hot nitrogen exiting said source of oxygen.
 21. The system ofclaim 20 wherein a heat exchanger is interposed between said air inletand said membrane of said source of oxygen, said heat exchangerreceiving heat from combustion products downstream of said gas generatorof said bottoming cycle, the combustion products routed past said heatexchanger in heat transfer relationship with air between said air inletand said membrane.
 22. A Brayton and Rankine combined cycle powergeneration system, comprising in combination: a gas compressor having aninlet and an outlet; a combustor downstream of said compressor, saidcombustor having a fuel port coupled to a source of fuel, an oxidizerport coupled to said compressor outlet and an outlet port for combustionproducts resulting from combustion of fuel from said source of fuel withoxidizer from said oxidizer port; a turbine downstream of saidcombustor, said turbine having an input coupled to said combustor outletport, an output for the combustion products entering said turbine atsaid input, and a power output; a heat recovery steam generatordownstream of said turbine output, said combustion products at leastpartially routed through said heat recovery steam generator; a gasgenerator including an oxygen inlet, a hydrocarbon fuel inlet and awater inlet, said water inlet receiving water from a water path in saidheat recovery steam generator in heat transfer contact with thecombustion products from said turbine output routed through said heatrecovery steam generator, such that the water entering said water inletof said gas generator is initially heated by the combustion productsfrom said turbine output at said heat recovery steam generator, said gasgenerator combusting the hydrocarbon fuel with oxygen to produce a hightemperature gas including carbon dioxide and water; a bottoming cycleturbine located downstream of said gas generator and receiving the hightemperature gas including carbon dioxide and water from said gasgenerator therein, said turbine outputting power from said system; and acondenser downstream of said bottoming cycle turbine, said condenser atleast partially separating carbon dioxide from water in the gas exitingsaid turbine.
 23. The system of claim 22 wherein a source of oxygen iscoupled to said oxygen inlet of said gas generator, said source ofoxygen including an air separation unit, such that the oxygen for thesource of oxygen is separated from air.
 24. The system of claim 23wherein said air separation unit includes an air inlet and an iontransfer membrane downstream from said air inlet with an oxygen outletdownstream of said membrane and a nitrogen rich outlet downstream ofsaid air inlet.
 25. The system of claim 24 wherein a heater is locatedbetween said air inlet and said membrane, said heater configured toreceive heat from combustion products downstream of said combustor. 26.The system of claim 24 wherein a heater is located between said airinlet and said membrane, said heater receiving heat transferred out ofthe combustion products passing through said heat recovery steamgenerator.
 27. The system of claim 24 wherein a heater is locatedbetween said air inlet and said membrane, said heater receiving heatfrom a high temperature gas including carbon dioxide and waterdischarged from said gas generator.
 28. The system of claim 23 whereinsaid source of oxygen feeds both said oxygen inlet of said gas generatorand said oxidizer port of said combustor through said gas compressor.29. The system of claim 28 wherein said air separation unit includes anair inlet and an ion transfer membrane downstream of said air inlet andan oxygen outlet downstream of said membrane, said oxygen outlet havinga heat exchanger adjacent thereto configured to heat water passingthrough said heat exchanger.
 30. The system of claim 29 wherein saidwater passing through said heat exchanger with oxygen from said oxygenoutlet is routed into a steam injection line coupled to said combustor.31. The system of claim 29 wherein said water passing through said heatexchanger is routed from said heat exchanger to said heat recovery steamgenerator for further heating of the water before entering said gasgenerator at said water inlet.
 32. The system of claim 28 wherein areturn duct is located downstream of said turbine, said return ductreceiving at least a portion of the combustion products passing throughsaid output of said turbine and extending to said inlet of saidcompressor, such that a semi-closed Brayton cycle is provided.
 33. Thesystem of claim 32 wherein a divider is located along said return duct,said divider dividing at least a portion of the combustion products intoa separation duct leading out of the system.
 34. The system of claim 33wherein said return duct includes a partial condenser therein includinga condensate line, said condensate line routed to said combustor along apath bypassing said compressor, such that at least a portion of waterentering said return duct is routed directly to said combustor, saidwater injection line passing through a heater between said partialcondenser and said combustor, said heater configured to receive heatfrom hot gases exiting said air separation unit.